Fig. 10. Schematic model of the genetic hierarchy leading to limb initiation in the zebrafish. Arrows indicate position within the hierarchy. FgfX refers to an Fgf signaling molecule acting in parallel to Fgf24 that is required for prdm1 expression. Bent arrow from prdm1 to fgf24 indicates a feedback mechanism operating between these two genes. fb, fin bud; IM, intermediate mesoderm; RA, retinoic acid; som, somites.